Negative electrode material, negative electrode sheet, lithium-ion battery and preparation method therefor

Abstract

The present application provides a negative electrode material, a negative electrode sheet, a lithium-ion battery and a preparation method therefor, so as to ameliorate the limitations caused by the limited amount of lithium difluorophosphate that is directly added to an electrolyte of a lithium-ion battery. The negative electrode material is used in a lithium-ion battery, which comprises lithium hexafluorophosphate. The negative electrode material comprises a negative electrode active material and aluminum oxide, wherein the aluminum oxide is attached to the surface of the negative electrode active material. In the negative electrode material, the mass ratio of the negative electrode active material to the aluminum oxide is 1:(0.001-0.03).

Description

Negative electrode materials, negative electrode sheets, lithium-ion batteries and their preparation methods

[0001] This application claims priority to Chinese Patent Application No. 2024119300425, filed with the Chinese Patent Office on December 25, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium-ion battery technology, specifically to a negative electrode material, a method for preparing a negative electrode material, a negative electrode sheet, a method for preparing a negative electrode sheet, a lithium-ion battery, and a method for preparing a lithium-ion battery. Background Technology

[0003] The electrode / electrolyte interface has a crucial impact on the performance of lithium-ion batteries, determining lithium-ion transport efficiency, cycle stability, and safety. Therefore, optimizing the properties of the electrode / electrolyte interface is one of the key approaches to improving lithium-ion battery performance.

[0004] To improve the performance of lithium-ion batteries, electrolyte additives can be used to enhance the properties of the electrode / electrolyte interface. Lithium difluorophosphate, a commonly used electrolyte additive, can reduce and decompose on the negative electrode surface to form a thermodynamically and mechanically stable solid electrolyte film. This not only inhibits further reduction and decomposition of the electrolyte at the negative electrode but also effectively improves cycle stability. Technical issues

[0005] Because lithium difluorophosphate has limited solubility in electrolytes, adding too much lithium difluorophosphate will cause the viscosity of the electrolyte to increase sharply, thereby reducing or even worsening the effect of performance improvement. Technical solutions

[0006] The embodiments of this application provide a negative electrode material, a negative electrode sheet, a lithium-ion battery, and a method for preparing the same, in order to overcome the limitations caused by the limited amount of lithium difluorophosphate that can be directly added to the electrolyte of a lithium-ion battery.

[0007] In a first aspect, embodiments of this application provide a negative electrode material for use in a lithium-ion battery, the lithium-ion battery comprising lithium hexafluorophosphate. The negative electrode material comprises a negative electrode active material and aluminum oxide, the aluminum oxide being attached to the surface of the negative electrode active material; in the negative electrode material, the mass ratio of the negative electrode active material to aluminum oxide is 1:(0.001~0.03).

[0008] Secondly, embodiments of this application provide a method for preparing a negative electrode material, used to prepare the negative electrode material provided in this application. The method for preparing the negative electrode material includes: mixing a negative electrode active material with alumina to obtain a mixture, and ball milling the mixture to allow alumina to adhere to the surface of the negative electrode active material, thereby obtaining the negative electrode material.

[0009] Thirdly, embodiments of this application provide a negative electrode sheet, including a negative electrode current collector and a negative electrode film layer disposed on the negative electrode current collector. The negative electrode film layer includes the negative electrode material provided in this application, or the negative electrode material prepared by the method for preparing the negative electrode material provided in this application.

[0010] Fourthly, embodiments of this application provide a method for preparing a negative electrode sheet, used to prepare the negative electrode sheet provided in this application. The method for preparing the negative electrode sheet includes: providing a negative electrode slurry, the negative electrode slurry comprising a negative electrode material and a solvent; performing a film-forming treatment on the negative electrode slurry on a negative electrode current collector to obtain a wet film layer; and performing a drying treatment on the wet film layer to obtain a negative electrode film layer.

[0011] Fifthly, embodiments of this application provide a lithium-ion battery, comprising: an electrode assembly and a casing, wherein a cavity is formed within the casing, the electrode assembly is disposed within the cavity, and an electrolyte is also contained within the cavity, the electrolyte wetting the electrode assembly, the electrolyte comprising lithium hexafluorophosphate, the electrode assembly comprising a positive electrode and a negative electrode disposed opposite to each other, the negative electrode being the negative electrode provided in this application, or a negative electrode prepared by the method for preparing the negative electrode provided in this application.

[0012] Sixthly, embodiments of this application provide a method for preparing a lithium-ion battery, comprising: providing an electrode assembly, the electrode assembly including a positive electrode and a negative electrode disposed opposite to each other, the negative electrode being the negative electrode provided in this application, or a negative electrode prepared by the method for preparing the negative electrode provided in this application; providing a housing, the housing having a cavity formed therein, and inserting the electrode assembly into the cavity; injecting an electrolyte into the cavity so that the electrolyte wets the electrode assembly to obtain a lithium-ion battery, wherein the electrolyte contains lithium hexafluorophosphate. Beneficial effects

[0013] In the negative electrode material provided in this application embodiment, aluminum oxide is attached to the surface of the negative electrode active material. When the negative electrode material is applied to a lithium-ion battery containing lithium hexafluorophosphate, the aluminum oxide can react with lithium hexafluorophosphate to form a coating layer containing lithium difluorophosphate and aluminum fluoride on the surface of the negative electrode active material, thereby effectively increasing the lithium difluorophosphate content in the lithium-ion battery. Since lithium difluorophosphate is generated in situ on the surface of the negative electrode active material in this application embodiment, it avoids adding too much lithium difluorophosphate to the electrolyte, which would increase the electrolyte viscosity and lead to poor electrolyte wettability on the electrode, resulting in battery performance degradation.

[0014] In addition, during the first charge, lithium difluorophosphate on the surface of the negative electrode active material will be reduced and decomposed and combine with aluminum fluoride to form a relatively stable composite solid electrolyte film. This not only inhibits the reduction and decomposition of the electrolyte on the negative electrode surface during cycling, but also improves the ionic conductivity of the solid electrolyte film at the interface between the negative electrode active material and the electrolyte, effectively reducing the electrode / electrolyte interface impedance and improving the rate performance and cycle stability of the battery. Attached Figure Description

[0015] Figure 1 is an ion chromatogram of the negative electrode sheet of the lithium-ion battery provided in Example 1 of this application before it is formed after being left to stand at high temperature. Embodiments of the present invention

[0016] In a first aspect, embodiments of this application provide a negative electrode material for use in a lithium-ion battery. Specifically, the negative electrode material is used in the negative electrode sheet of a lithium-ion battery. The lithium-ion battery comprises lithium hexafluorophosphate (LiPF6). As an example, the lithium-ion battery includes an electrolyte in which lithium hexafluorophosphate is contained.

[0017] Specifically, the negative electrode material includes a negative electrode active material and alumina, with the alumina adhering to the surface of the negative electrode active material. In the negative electrode material, the mass ratio of the negative electrode active material to alumina is 1:(0.001~0.03).

[0018] In the negative electrode material provided in this application embodiment, aluminum oxide is attached to the surface of the negative electrode active material. When the negative electrode material is applied to a lithium-ion battery containing lithium hexafluorophosphate, the aluminum oxide can react with lithium hexafluorophosphate to form a coating layer containing lithium difluorophosphate and aluminum fluoride on the surface of the negative electrode active material, thereby effectively increasing the lithium difluorophosphate content in the lithium-ion battery. Since lithium difluorophosphate is generated in situ on the surface of the negative electrode active material in this application embodiment, it avoids adding too much lithium difluorophosphate to the electrolyte, which would increase the electrolyte viscosity and lead to poor electrolyte wettability on the electrode, resulting in battery performance degradation.

[0019] During the first charge, lithium difluorophosphate on the surface of the negative electrode active material will be reduced and decomposed and combine with aluminum fluoride to form a relatively stable composite solid electrolyte film. This not only inhibits the reduction and decomposition of the electrolyte on the negative electrode surface during cycling, but also improves the ionic conductivity of the solid electrolyte film at the interface between the negative electrode active material and the electrolyte, effectively reducing the electrode / electrolyte interface impedance and improving the rate performance and cycle stability of the battery.

[0020] The alumina content in the negative electrode material usually affects the amount of lithium difluorophosphate generated in the lithium-ion battery reaction. Therefore, the alumina content should not be too low. However, alumina (Al2O3) has poor conductivity. If the alumina content in the negative electrode material is too high, the conductivity of the negative electrode material will decrease, thereby increasing the internal resistance of the lithium-ion battery. In addition, the high alumina content will also lead to increased consumption of lithium hexafluorophosphate, which will have an adverse effect on the cycle life of the lithium-ion battery.

[0021] By controlling the content of alumina in the negative electrode material, the mass ratio of the negative electrode active material to alumina is 1:(0.001~0.03). Within this range, the electrochemical performance of the lithium-ion battery is better.

[0022] As an example, in the negative electrode material, the mass ratio of the negative electrode active material to alumina is 1:0.001, 1:0.002, 1:0.003, 1:0.004, 1:0.005, 1:0.006, 1:0.007, 1:0.008, 1:0.009, 1:0.01, 1:0.012, 1:0.014, 1:0.016, 1:0.018, 1:0.02, 1:0.022, 1:0.024, 1:0.026, 1:0.028, or 1:0.03.

[0023] In some embodiments, the volume fraction of alumina particles with a diameter of 10 nm to 30 nm is greater than or equal to 20 vol%. It can be understood that the alumina in the negative electrode material is granular, i.e., alumina particles. Based on volume content, at least 20 vol% of the alumina particles have a diameter of 10 nm to 30 nm. Generally, the smaller the particle size of the alumina in the negative electrode material, the easier it is for the alumina to react with lithium hexafluorophosphate to form lithium difluorophosphate. However, since alumina adheres to the surface of the negative electrode active material, excessively small alumina particle size can actually make it more difficult for the electrolyte to penetrate into the interior of the negative electrode active material, affecting mass transfer, thereby reducing the probability of contact between alumina and lithium hexafluorophosphate and reducing the amount of lithium difluorophosphate generated. As an example, the alumina particles have a diameter of 10 nm, 15 nm, 20 nm, 25 nm, or 30 nm. As an example, the volume percentage of alumina particles with a diameter of 10nm to 30nm in the alumina is 20vol%, 30vol%, 40vol%, 50vol%, 60vol%, 70vol%, 80vol%, 90vol%, or 100vol%. Furthermore, excessively small alumina particle size in the anode material will increase the production cost of the anode material.

[0024] In some embodiments, the negative electrode active material includes at least one of graphite, soft carbon, hard carbon, silicon, and silicon oxide. It should be noted that soft carbon and hard carbon are two different carbon materials, primarily differing in their graphitization ability at high temperatures. Soft carbon can be graphitized at temperatures above 2500°C, while hard carbon is difficult to graphitize. Soft carbon typically has a low and stable charge / discharge potential plateau, resulting in large charge / discharge capacity and high efficiency, while hard carbon has a stable structure and a long charge / discharge cycle life.

[0025] In some embodiments, the chemical formula for silicon oxide is SiO. x And 0 < x < 2. Understandably, the silicon oxide here refers to an incomplete oxide of silicon. Here, x represents the oxygen content in the silicon oxide, and x can be any value between 0 and 2, such as 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 1.99, etc. As an example, silicon oxide includes silicon suboxide.

[0026] Secondly, embodiments of this application also provide a method for preparing a negative electrode material, which is used to prepare the negative electrode material provided in this application.

[0027] Specifically, the preparation methods of the negative electrode material include:

[0028] S11. Mix the negative electrode active material with alumina to obtain a mixture;

[0029] S12. The mixture is ball-milled to allow alumina to adhere to the surface of the negative electrode active material, thus obtaining the negative electrode material.

[0030] The method for preparing the negative electrode material provided in this application is simple to operate, easy to implement, and has no special environmental control requirements.

[0031] In some embodiments, the mixture of the negative electrode active material and alumina is obtained by dry mixing of the solid particles of the negative electrode active material and the solid particles of alumina. Of course, in other embodiments, a wet mixing method can also be used. Here, "dry method" refers to mixing without using a solvent, while "wet method" refers to mixing with a solvent. Compared to the wet method, the dry method is simpler, eliminating the need for further drying to remove the solvent, thus simplifying the process and reducing production costs.

[0032] In some embodiments, the ball milling time is 3 to 10 hours, and the rotation speed is 80 to 150 r / min. Within this parameter range, alumina can be uniformly coated on the surface of the negative electrode active material. As an example, the ball milling time is 3, 4, 5, 6, 7, 8, 9, or 10 hours, and the rotation speed is 80, 90, 100, 110, 120, 130, 140, or 150 r / min.

[0033] Thirdly, embodiments of this application also provide a negative electrode sheet, which includes a negative electrode current collector and a negative electrode film layer, the negative electrode film layer being disposed on the negative electrode current collector. The negative electrode film layer includes the negative electrode material provided in this application, or the negative electrode material prepared by the method for preparing the negative electrode material provided in this application.

[0034] In some embodiments, the aluminum content in the negative electrode film is between 1800 ppm and 56000 ppm. Within this range, the amount of lithium difluorophosphate generated in the lithium-ion battery reaction can be balanced with the conductivity of the negative electrode, and the consumption of lithium hexafluorophosphate can also be reduced. For example, the aluminum content in the negative electrode film is 1800 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 6000 ppm, 8000 ppm, 10000 ppm, 20000 ppm, 30000 ppm, 40000 ppm, 50000 ppm, or 56000 ppm.

[0035] In some embodiments, the negative electrode film layer further includes a binder, a conductive agent, and a thickener. In the negative electrode film layer, the mass ratio of the negative electrode material, binder, conductive agent, and thickener is (95~98):(0.8~2.5):(0.5~3.5):(0.7~2.0). As an example, the conductive agent includes at least one selected from conductive carbon black, carbon fiber, carbon nanotubes (CNTs), and graphene. As an example, the conductive carbon black includes at least one selected from acetylene black, Super P (SP), and Ketjen black. As an example, the carbon nanotubes include at least one selected from single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). As an example, the binder includes at least one selected from polyacrylic acid (PAA) and polystyrene-butadiene copolymer (SBR). As an example, the thickener includes carboxymethyl cellulose (CMC).

[0036] In some embodiments, the negative current collector is a copper foil with a thickness of 4 μm to 12 μm.

[0037] Fourthly, embodiments of this application also provide a method for preparing a negative electrode sheet, which is used to prepare the negative electrode sheet provided in this application.

[0038] Specifically, the preparation method of the negative electrode sheet includes:

[0039] S21. Provide a negative electrode slurry, which includes a negative electrode material and a solvent;

[0040] S22. The negative electrode slurry is subjected to film formation treatment on the negative electrode current collector to obtain a wet film layer;

[0041] S23. The wet film layer is dried to obtain the negative electrode film layer.

[0042] The method for preparing the negative electrode sheet provided in this application is simple to operate, easy to implement, and has no special environmental control requirements.

[0043] In some embodiments, the solvent in the negative electrode slurry includes one of deionized water and N-methylpyrrolidone (NMP).

[0044] In some embodiments, the negative electrode slurry is deposited on the negative electrode current collector to obtain a wet film layer. Specifically, this involves coating the negative electrode slurry onto the negative electrode current collector to obtain a wet film layer. It is understood that the wet film layer contains a solvent. Optionally, the coating method includes, but is not limited to, at least one of gravure coating, microgravure coating, spray coating, and electrospinning techniques.

[0045] The drying process for the wet film layer specifically involves removing the solvent from the wet film layer.

[0046] In some embodiments, the drying process is vacuum high-temperature drying, with the temperature ranging from 60°C to 100°C. For example, the vacuum high-temperature drying temperature may be 60°C, 70°C, 80°C, 90°C, or 100°C.

[0047] Fifthly, embodiments of this application provide a lithium-ion battery, which includes an electrode assembly and a casing. A cavity is formed within the casing, and the electrode assembly is disposed within the cavity. An electrolyte is also contained within the cavity, and the electrolyte wets the electrode assembly. The electrolyte comprises lithium hexafluorophosphate. The electrode assembly includes a positive electrode and a negative electrode disposed opposite to each other. The negative electrode is the negative electrode provided in this application, or a negative electrode prepared by the method for preparing the negative electrode provided in this application.

[0048] In some embodiments, the molar ratio of lithium hexafluorophosphate to aluminum oxide in the lithium-ion battery is 1:(0.005~0.15). If the molar ratio of lithium hexafluorophosphate to aluminum oxide is too high, i.e., the aluminum oxide content in the negative electrode material is too low, it will lead to a decrease in the amount of lithium difluorophosphate generated in the lithium-ion battery reaction. However, if the molar ratio of lithium hexafluorophosphate to aluminum oxide is too low, i.e., the aluminum oxide content in the negative electrode material is too high, it will lead to an increase in the consumption of lithium hexafluorophosphate. As an example, in lithium-ion batteries, the molar ratio of lithium hexafluorophosphate to aluminum oxide is 1:0.005, 1:0.006, 1:0.007, 1:0.008, 1:0.009, 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.10, 1:0.11, 1:0.12, 1:0.13, 1:0.14, or 1:0.15.

[0049] In some embodiments, the electrolyte further comprises an organic solvent in which lithium hexafluorophosphate is dissolved. The organic solvent includes at least one of ethylene carbonate (EC), methyl propyl carbonate (MSDS), propylene carbonate (PC), butenyl carbonate (CBC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC).

[0050] In some embodiments, the electrode assembly further includes a separator disposed between the positive and negative electrode plates to separate them and prevent short circuits caused by contact between the positive and negative electrode plates. As an example, the separator includes at least one of a polyethylene (PE) film and a polypropylene (PP) film.

[0051] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on the positive current collector. As an example, the positive electrode film layer contains a positive electrode active material. As an example, the positive electrode active material includes at least one selected from lithium nickel cobalt manganese oxide (NMC), lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4, LFP), lithium manganese oxide (LiMn2O4, LMO), and lithium nickel cobalt aluminum oxide (NCA). Optionally, the positive electrode film layer also includes a conductive agent, which includes at least one selected from carbon black, conductive graphite, graphene, and carbon nanotubes. Optionally, the positive electrode film layer also includes a binder, which includes at least one organic polymer such as polyacrylic acid (PAA) and polyvinylidene fluoride (PVDF).

[0052] In some embodiments, the positive current collector is an aluminum foil with a thickness of 6 μm to 15 μm.

[0053] Sixthly, embodiments of this application provide a method for preparing a lithium-ion battery, comprising:

[0054] S31. Provide an electrode assembly, which includes a positive electrode and a negative electrode arranged opposite to each other. The negative electrode is the negative electrode provided in this application, or a negative electrode prepared by the method for preparing the negative electrode provided in this application.

[0055] S32. Provide a housing with a cavity formed inside the housing, and install the electrode assembly into the cavity;

[0056] S33. Inject electrolyte into the cavity so that the electrolyte wets the electrode assembly to obtain a lithium-ion battery, wherein the electrolyte contains lithium hexafluorophosphate.

[0057] The lithium-ion battery preparation method provided in this application is simple to operate, easy to implement, and has no special environmental control requirements.

[0058] In some embodiments, the method for preparing a lithium-ion battery further includes:

[0059] S34. Perform a settling treatment on the lithium-ion battery. The reaction rate between lithium hexafluorophosphate and alumina is relatively slow. By performing a settling treatment on the lithium-ion battery obtained after electrolyte injection, lithium hexafluorophosphate can react with effective alumina during the settling process and generate a sufficient amount of lithium difluorophosphate.

[0060] In some embodiments, the settling temperature is 40°C to 80°C. Increasing the settling temperature can accelerate the reaction rate between lithium hexafluorophosphate and alumina; however, the settling temperature should not be too high, otherwise it will affect the stability of other structures or components in the lithium-ion battery. For example, the stability of lithium hexafluorophosphate decreases at high temperatures, and it is prone to decomposition. As examples, the settling temperatures are 40°C, 50°C, 60°C, 70°C, or 80°C.

[0061] In some implementations, the settling time is 12h to 96h. Controlling the settling time ensures sufficient reaction between lithium hexafluorophosphate and alumina. Generally, increasing the settling temperature shortens the settling time, thereby increasing production efficiency. Examples of settling times include 12h, 20h, 30h, 40h, 50h, 60h, 70h, 80h, 90h, or 96h.

[0062] In some embodiments, the method for preparing a lithium-ion battery further includes:

[0063] S35. After the settling process, the lithium-ion battery undergoes a formation process.

[0064] During the formation process, lithium difluorophosphate on the surface of the negative electrode active material is reduced and decomposed and combines with aluminum fluoride to form a relatively stable composite solid electrolyte film. This not only inhibits the reduction and decomposition of the electrolyte on the negative electrode surface during cycling, but also improves the ionic conductivity of the solid electrolyte film at the interface between the negative electrode active material and the electrolyte, effectively reducing the electrode / electrolyte interface impedance and improving the rate performance and cycle stability of the battery.

[0065] In some implementations, the lithium-ion battery is a liquid battery.

[0066] The following description is based on specific embodiments.

[0067] Example 1

[0068] 1. Preparation of negative electrode materials:

[0069] The negative electrode active material, graphite, and alumina (both are solid particles) were mixed at a mass ratio M of 1:0.01 and then ball-milled for 5 hours at a speed of 100 r / min. This resulted in the alumina uniformly coating the surface of the negative electrode active material, with the volume percentage of solid particles with a particle size of 20 nm in the alumina being 50 vol%.

[0070] 2. Preparation of negative electrode sheet:

[0071] The prepared negative electrode material, binder SBR, thickener CMC, and conductive agent SP are added to deionized water as a solvent and stirred to disperse evenly, thus obtaining a negative electrode slurry. The negative electrode slurry is coated onto a copper foil current collector and dried under vacuum at high temperature to obtain a negative electrode film layer bonded to the current collector, thereby obtaining a negative electrode sheet. The mass ratio of negative electrode material, binder, thickener, and conductive agent in the negative electrode film layer of the negative electrode sheet is 96:2.0:1.3:0.7, and the vacuum high-temperature drying temperature is 90℃ for 12 hours.

[0072] 3. Lithium-ion battery manufacturing:

[0073] The prepared negative electrode sheet is stacked with the corresponding positive electrode sheet and separator in the order of positive electrode sheet-separator-negative electrode sheet, so that the separator is placed between the positive and negative electrode sheets to isolate them, thus obtaining a bare cell. The bare cell is placed in a shell and a fixed amount of prepared electrolyte (1.2M LiPF6, solvent including EC, DMC and EMC, where the volume ratio of EC:DMC:EMC = 4:3:3) is injected before encapsulation to obtain a lithium-ion battery after electrolyte injection. The separator is a PE film, and the composition of the positive electrode sheet is LFP:PVDF:CNT:SP = 97:1.8:0.5:0.7.

[0074] After being injected with electrolyte, the lithium-ion battery was placed at a high temperature of 60°C for 48 hours. This allowed the aluminum oxide on the surface of the negative electrode active material to react with the lithium hexafluorophosphate in the electrolyte, forming a coating layer containing lithium difluorophosphate and aluminum fluoride on the surface of the negative electrode active material. The molar ratio N of lithium hexafluorophosphate to aluminum oxide was 1:0.05.

[0075] After being left to stand at high temperatures, the lithium-ion batteries undergo formation and capacity testing to produce the final product.

[0076] Example 2

[0077] The only difference from Example 1 is that, in the preparation of the negative electrode material, the negative electrode active material graphite and alumina are in a mass ratio M of 1:0.001, and the molar ratio N of lithium hexafluorophosphate and alumina in the lithium-ion battery is 1:0.005.

[0078] Everything else is the same as in Example 1.

[0079] Example 3

[0080] The only difference from Example 1 is that, in the preparation of the negative electrode material, the negative electrode active material graphite and alumina are in a mass ratio M of 1:0.005, and the molar ratio N of lithium hexafluorophosphate and alumina in the lithium-ion battery is 1:0.025.

[0081] Everything else is the same as in Example 1.

[0082] Example 4

[0083] The only difference from Example 1 is that, in the preparation of the negative electrode material, the negative electrode active material graphite and alumina are in a mass ratio M of 1:0.008, and the molar ratio N of lithium hexafluorophosphate and alumina in the lithium-ion battery is 1:0.04.

[0084] Everything else is the same as in Example 1.

[0085] Example 5

[0086] The only difference from Example 1 is that, in the preparation of the negative electrode material, the negative electrode active material graphite and alumina are in a mass ratio M of 1:0.012, so the molar ratio N of lithium hexafluorophosphate and alumina in the lithium-ion battery is 1:0.06.

[0087] Everything else is the same as in Example 1.

[0088] Example 6

[0089] The only difference from Example 1 is that, in the preparation of the negative electrode material, the negative electrode active material graphite and alumina are in a mass ratio M of 1:0.03, so the molar ratio N of lithium hexafluorophosphate and alumina in the lithium-ion battery is 1:0.15.

[0090] Everything else is the same as in Example 1.

[0091] Example 7

[0092] The only difference from Example 4 is that silicon is used instead of graphite in the preparation of the negative electrode material.

[0093] Everything else is the same as in Example 4.

[0094] Example 8

[0095] The only difference from Example 5 is that silicon is used instead of graphite in the negative electrode active material during the preparation of the negative electrode material.

[0096] Everything else is the same as in Example 5.

[0097] Example 9

[0098] The only difference from Example 1 is that the high-temperature settling temperature during lithium-ion battery preparation is 40°C.

[0099] Everything else is the same as in Example 1.

[0100] Example 10

[0101] The only difference from Example 1 is that the high-temperature settling temperature during lithium-ion battery preparation is 80°C.

[0102] Everything else is the same as in Example 1.

[0103] Example 11

[0104] The only difference from Example 1 is that the volume percentage of solid particles with a diameter of 10 nm in the alumina is 50 vol.

[0105] Everything else is the same as in Example 1.

[0106] Example 12

[0107] The only difference from Example 1 is that the volume percentage of solid particles with a diameter of 30 nm in the alumina is 50 vol.

[0108] Everything else is the same as in Example 1.

[0109] Example 13

[0110] The only difference from Example 1 is that the volume percentage of solid particles with a diameter of 20 nm in the alumina is 20 vol.

[0111] Everything else is the same as in Example 1.

[0112] Example 14

[0113] The only difference from Example 1 is that the volume percentage of solid particles with a diameter of 20 nm in the alumina is 90 vol.

[0114] Everything else is the same as in Example 1.

[0115] Comparative Example 1

[0116] The only difference from Example 1 is that no aluminum oxide is added during the preparation of the negative electrode material. That is, the mass ratio M of the negative electrode active material graphite to aluminum oxide is 1:0. Thus, during the preparation of the lithium-ion battery, the molar ratio N of lithium hexafluorophosphate in the electrolyte to aluminum oxide in the negative electrode is also 1:0.

[0117] Comparative Example 2

[0118] The only difference from Example 1 is that, in the preparation of the negative electrode material, the negative electrode active material graphite and alumina are in a mass ratio M of 1:0.0005, and the molar ratio N of lithium hexafluorophosphate and alumina in the lithium-ion battery is 1:0.0025.

[0119] Everything else is the same as in Example 1.

[0120] Comparative Example 3

[0121] The only difference from Example 1 is that, in the preparation of the negative electrode material, the negative electrode active material graphite and alumina are in a mass ratio M of 1:0.04, so the molar ratio N of lithium hexafluorophosphate and alumina in the lithium-ion battery is 1:0.20.

[0122] Everything else is the same as in Example 1.

[0123] Comparative Example 4

[0124] The only difference from Example 1 is that silicon is used instead of graphite in the preparation of the negative electrode material, and aluminum oxide is not added. That is, the mass ratio M of silicon to aluminum oxide in the negative electrode active material is 1:0. In this way, the molar ratio N of lithium hexafluorophosphate in the electrolyte to aluminum oxide in the negative electrode is also 1:0 in the preparation of the lithium-ion battery.

[0125] Table 1

[0126] Group | Anode Active Material | Mass Ratio of Anode Active Material to Alumina | Molar Ratio of Lithium Hexafluorophosphate to Alumina | Standing Temperature | Alumina Particle Size | Example 1 | Graphite 1:0.01 | 1:0.05 | 60℃ | 50 vol% | 20 nm | Example 2 | Graphite 1:0.001 | 1:0.005 | 60℃ | 50 vol% | 20 nm | Example 3 | Graphite 1:0.005 | 1:0.025 | 60℃ | 50 vol% | 20 nm | Example 4 ...1 | 1:0.005 | 60℃ | 50 vol% | 20 nm | Example 5: Graphite 1:0.0081:0.04, 60℃, 50 vol%, 20 nm. Example 6: Graphite 1:0.031:0.15, 60℃, 50 vol%, 20 nm. Example 7: Silicon 1:0.0081:0.04, 60℃, 50 vol%, 20 nm. Example 8: Silicon 1:0.0121:0.06, 60℃, 50 vol%. Example 9: Graphite 1:0.011:0.05, 40℃, 50 vol. % is 20 nm. Example 10: Graphite 1:0.011:0.05, 80℃, 50 vol. % is 20 nm. Example 11: Graphite 1:0.011:0.05, 60℃, 50 vol. % is 10 nm. Example 12: Graphite 1:0.011:0.05, 60℃, 50 vol. % is 30 nm. Example 13: Graphite 1:0.011... Example 14: Graphite 1:0.01 1:0.05 60℃ 90 vol% 20 nm Comparative Example 1: Graphite 1:0 1:0 60℃ / Comparative Example 2: Graphite 1:0.0005 1:0.0025 60℃ 50 vol% 20 nm Comparative Example 3: Graphite 1:0.04 1:0.20 60℃ 50 vol% 20 nm Comparative Example 4: Silicon 1:0 1:0 60℃ /

[0127] The following tests were performed on each embodiment and comparison, and the test results are recorded in Table 2:

[0128] 1. Test the aluminum content in the negative electrode film layer of the negative electrode sheet. The test methods include:

[0129] Weigh 0.5g of the negative electrode film sample, grind it into a fine powder, place it in a microwave digestion vessel, add 3ml of nitric acid and 9ml of hydrochloric acid, heat to 200℃ for 30min, cool, filter, transfer the filtrate to a 100ml volumetric flask, and dilute with deionized water to the mark line. Introduce the prepared solution into an inductively coupled plasma spectrometer (ICP spectrometer) for analysis, record the detection intensity or percentage content, use a standard curve to convert the sample signal intensity into elemental concentration, and calculate the aluminum content in the sample.

[0130] 2. Ion chromatography was performed on the negative electrode of the lithium-ion battery before it was formed after being left to stand at high temperature. The test methods included:

[0131] A 2cm x 2cm negative electrode sheet, after being allowed to stand at high temperature, was immersed in 10ml of acetonitrile for 4 hours. The immersion solution was filtered to obtain a filtrate. 1ml of the filtrate was diluted 10 times, and 5ml of the diluted solution was tested by ion chromatography to obtain the peak intensity of the characteristic peak (representing lithium difluorophosphate) from 7min to 9min. The detection conditions for ion chromatography were as follows: an anion exchange column, a mobile phase of 3.6mmol / L sodium carbonate solution (solvent being 40% acetonitrile aqueous solution), and a conductivity detector.

[0132] 3. Test the 3C rate discharge capacity retention rate of lithium-ion batteries. The test methods include:

[0133] The lithium-ion battery was charged to 100% SOC (State of Charge) at 0.1C at 25℃, and then discharged to 0% SOC at 0.1C. The discharge capacity C1 was recorded. Then the battery was charged to 100% SOC at 0.1C, and then discharged to 0% SOC at 3C. The discharge capacity C2 was recorded. The 3C rate discharge capacity retention rate = C2 / C1.

[0134] 4. Test the cycle performance of lithium-ion batteries. Test methods include:

[0135] At 45°C, the lithium-ion battery was subjected to 500 constant current charge-discharge cycles at a rate of 1.0C (nominal capacity). The capacity retention rate (C%) after 500 cycles = (discharge capacity of the 500th cycle / initial discharge capacity) × 100%.

[0136] Table 2

[0137] Group Aluminum content (ppm) Characteristic peak intensity (μS·cm⁻¹) 7min-9min 3C rate discharge capacity retention (%) 500-cycle capacity retention (%) Example 1 1973 492 74.9 90.1 Example 2 2043 236 9.5 86.9 Example 3 9950 537 1.4 88.9 Example 4 1589 676 72.3 89.5 Example 5 2432 510 678.6 90.6 Example 6 589 642 438 0.3 91.1 Example 7 1507 875 75.6 84.4 Example 8 2252810677.385.7 Example 9 199647872.689.5 Example 10 198968273.489.6 Example 11 199129375.290.3 Example 12 198569174.689.8 Example 13 198789074.389.3 Example 14 199699375.690.6 Comparative Example 1 / / 65.385.5 Comparative Example 2 9851965.585.8 Comparative Example 3 7968332067.586.0 Comparative Example 4 / / 67.881.3

[0138] Figure 1 is an ion chromatogram of the negative electrode of the lithium-ion battery provided in Example 1 before its formation after being placed at high temperature. As can be seen from Figure 1, there is a distinct characteristic peak at the position of 7 min to 9 min in the spectrum, which indicates that lithium difluorophosphate is present in the negative electrode of the lithium-ion battery. Since lithium difluorophosphate was not added during the preparation of the lithium-ion battery, the lithium difluorophosphate was mainly generated by the reaction of alumina in the negative electrode and lithium hexafluorophosphate in the electrolyte during the high-temperature placement of the lithium-ion battery.

[0139] As shown in Table 1, as the mass ratio of negative electrode active material to alumina in the negative electrode material decreases, i.e., the alumina content in the negative electrode material increases (in order: Comparative Example 1, Comparative Example 2, Examples 2 to 4, Example 1, Examples 5 to 6, Comparative Example 3), the aluminum content in the negative electrode film increases. Since the amount of electrolyte injected in each example is the same during the preparation of the lithium-ion battery, the molar ratio of lithium hexafluorophosphate to alumina in the lithium-ion battery also decreases, i.e., the amount of alumina increases. After the lithium-ion battery is subjected to high-temperature settling, the concentration of lithium difluorophosphate in the negative electrode increases. However, the 3C rate discharge capacity retention rate and the capacity retention rate after 500 cycles of the lithium-ion battery show an initial increase followed by a decrease. This is because during the first charge, lithium difluorophosphate will be reduced and decomposed and combine with aluminum fluoride to form a relatively thin layer. A stable composite solid electrolyte membrane exhibits improved stability as the mass ratio of the negative electrode active material to alumina decreases and the concentration of lithium difluorophosphate increases, resulting in enhanced thickness and density. This allows the composite solid electrolyte membrane to effectively suppress the reduction and decomposition of the electrolyte on the negative electrode surface during cycling, thereby improving the cycle performance of the lithium-ion battery. Furthermore, the ionic conductivity of the solid electrolyte membrane also increases, reducing the electrode / electrolyte interfacial impedance and improving the rate performance of the lithium-ion battery. However, an excessively low mass ratio of the negative electrode active material to alumina leads to an excessively thick solid electrolyte membrane, which in turn increases the electrode / electrolyte interfacial impedance and reduces the rate performance of the lithium-ion battery. Conversely, an excessive alumina content can cause excessive consumption of lithium hexafluorophosphate in the electrolyte, further degrading the cycle performance of the lithium-ion battery.

[0140] Comparing Comparative Example 4, Example 7, and Example 8, it can be seen that when the negative electrode active material is silicon, using alumina to coat silicon can also improve the cycle performance and rate performance of lithium-ion batteries; however, compared with when the negative electrode active material is graphite, the cycle performance and rate performance of the ion battery are slightly worse. This is because silicon is more prone to expansion than graphite, thus affecting the performance of the ion battery.

[0141] Comparing Examples 9, 1, and 10, it can be seen that as the resting temperature increases during high-temperature resting, the concentration of lithium difluorophosphate first increases and then decreases. The 3C rate discharge capacity retention rate and the capacity retention rate after 500 cycles of the lithium-ion battery also show a pattern of first increasing and then decreasing. This is because as the resting temperature increases, the reaction between alumina and lithium hexafluorophosphate can be accelerated to generate lithium difluorophosphate. However, excessively high resting temperatures can lead to instability of lithium hexafluorophosphate, thereby affecting the amount of lithium difluorophosphate generated by the reaction between alumina and lithium hexafluorophosphate, and ultimately affecting the performance of the lithium-ion battery.

[0142] Comparing Examples 11, 1, and 12, it can be seen that the aluminum content in the negative electrode film layer of the negative electrode sheet is relatively similar. However, as the particle size of alumina increases, the concentration of lithium difluorophosphate in the negative electrode sheet decreases sequentially after the lithium-ion battery is subjected to high-temperature settling. This is because as the particle size of alumina increases, the specific surface area of ​​alumina decreases, which leads to a decrease in the probability of alumina molecules contacting lithium hexafluorophosphate molecules, thereby reducing the amount of lithium difluorophosphate generated, and ultimately causing a decrease in the rate performance and cycle performance of the lithium-ion battery.

[0143] Comparing Examples 13, 1, and 14, it can be seen that the aluminum content in the negative electrode film layer of the negative electrode sheet is relatively similar. However, as the volume fraction of alumina with a particle size of 20 nm increases, the concentration of lithium difluorophosphate in the negative electrode sheet increases sequentially after the lithium-ion battery is subjected to high-temperature settling. This is because when the alumina particle size is 20 nm, a sufficiently large gap can be formed between the alumina particles to ensure that the electrolyte containing lithium hexafluorophosphate effectively penetrates and wets the alumina. In addition, the specific surface area of ​​alumina particles of this size is large enough. The dual effect increases the probability of contact between alumina molecules and lithium hexafluorophosphate molecules. As the content of alumina particles with a particle size of 20 nm increases, the probability of contact between alumina molecules and lithium hexafluorophosphate molecules further increases, thereby increasing the amount of lithium difluorophosphate generated and ultimately improving the rate performance and cycle performance of the lithium-ion battery.

Claims

1. A negative electrode material for a lithium ion battery, the lithium ion battery comprising lithium hexafluorophosphate, the negative electrode material comprising a negative electrode active material and aluminum oxide, the aluminum oxide being attached to a surface of the negative electrode active material; a mass ratio of the negative electrode active material to the aluminum oxide in the negative electrode material being 1 :( 0.001-0.03).

2. The negative electrode material of claim 1, wherein, A volume fraction of aluminum oxide particles with a particle size of 10-30 nm in the aluminum oxide is greater than or equal to 20 vol%.

3. The negative electrode material of any one of claims 1-2, wherein, The negative electrode active material comprises at least one of graphite, soft carbon, hard carbon, silicon, and silicon oxide.

4. A method for preparing a negative electrode material according to any one of claims 1 to 3, comprising: The negative electrode active material and the aluminum oxide are mixed to obtain a mixture, and the mixture is subjected to ball milling treatment so that the aluminum oxide is attached to the surface of the negative electrode active material, thereby obtaining the negative electrode material.

5. The method of producing a negative electrode material according to claim 4, wherein The ball milling treatment is performed for 3-10 h at a rotation speed of 80-150 r / min. 6.A negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer disposed on the negative electrode current collector, the negative electrode film layer comprising the negative electrode material according to any one of claims 1-3, or the negative electrode material prepared by the preparation method according to any one of claims 4-5.

7. The negative electrode sheet according to claim 6, wherein A content of aluminum in the negative electrode film layer is 1800-56000 ppm. 8.The negative electrode sheet according to any one of claims 6-7, the negative electrode film layer further comprising a binder, a conductive agent, and a thickening agent, a mass ratio of the negative electrode material, the binder, the conductive agent, and the thickening agent in the negative electrode film layer being (95-98) :( 0.8-2.5) :( 0.5-3.5) :( 0.7-2.0).

9. A method for producing a negative electrode sheet for producing the negative electrode sheet according to any one of claims 6 to 8, comprising: A negative electrode slurry is provided, the negative electrode slurry comprising a negative electrode material and a solvent; the negative electrode slurry is subjected to film forming treatment on a negative electrode current collector to obtain a wet film layer; and the wet film layer is subjected to drying treatment to obtain a negative electrode film layer.

10. The method of producing a negative electrode sheet according to claim 9, wherein The drying treatment is vacuum high-temperature drying, and a temperature of the vacuum high-temperature drying is 60-100 ℃.

11. A lithium-ion battery comprising: An electrode assembly and a shell, the shell forming a container cavity therein, the electrode assembly being disposed in the container cavity, and an electrolyte being loaded in the container cavity, the electrolyte infiltrating the electrode assembly, the electrolyte comprising lithium hexafluorophosphate, the electrode assembly comprising oppositely disposed positive electrode sheets and negative electrode sheets, the negative electrode sheets being the negative electrode sheet according to any one of claims 6-8, or the negative electrode sheet prepared by the preparation method according to any one of claims 9-10.

12. The lithium-ion battery of claim 11, wherein, In the lithium ion battery, a molar ratio of the lithium hexafluorophosphate to the aluminum oxide is 1 :( 0.005-0.15). 13.The lithium ion battery according to any one of claims 11-12, the electrolyte further comprising an organic solvent, the organic solvent comprising at least one of ethylene carbonate, methyl propyl carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. 14.A preparation method of a lithium ion battery, comprising: Provided are an electrode assembly, a lithium ion battery, and a preparation method of the lithium ion battery. Provided are an electrode assembly, a lithium ion battery, and a preparation method of the lithium ion battery.

15. The method of claim 14, further comprising: The lithium ion battery is subjected to a standing treatment.

16. The method of producing a lithium-ion battery according to any one of claims 14-15, wherein, The standing temperature of the standing treatment is 40-80°C.

17. The method of producing a lithium-ion battery according to any one of claims 14-16, wherein, The standing time of the standing treatment is 12-96 hours.

18. The method of claim 14-17, further comprising: The lithium ion battery is subjected to a formation treatment after the standing treatment.

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